The research proposed here is an attempt to test, extend, and if possible improve on the psychosocial theory of problem behavior recently advanced by Richard and Shirley Jessor. The Jessors have devised measures of personality and proximal perceived environment which together are capable of explaining large proportions of observed variance in self-reported marijuana use and alcohol use by high school and college students. In the present research plan a rationale is offered to show that self-monitoring (i.e., the extent to which one's behavior is susceptible to the influence of immediate, situation-specific social cues) may be an affective moderator variable in the study of marijuana use, drinking, and cigarette smoking. A pilot study with college students revealed nonchance patterns of relationship between self-monitoring and each of these behaviors. With respect to marijuana use and tobacco use, the pattern for freshmen was different from that for upperclassmen, suggesting that developmental phenomena may affect the presumed interaction of self-monitoring with personality and perceived environment variables in the prediction of marijuana behavior. Three successive cohorts of college freshmen will be assessed on two occasions one year apart. On each occasion each subject will complete the self-monitoring scale, the six predictors measures which were most effective in the Jessors' college study, three additional predictor measures, and questions concerning use of marijuana, alcohol, and tobacco. Pearson coefficients, hierarchical multiple regression, and step-wise multiple regression will be used to examine efficacy of predictors with respect to each behavior on each occasion for the total cohort and for subjects segregated on the basis of self-monitoring score. Cells of the cohort X occasion matrix will be compared in sets of three to identify significant longitudinal, cross-sectional, and time-lag differences. Combinations of these differences will then be interpreted as indications of age or cohort or time of testing effects on the three behaviors, on each predictor and its contributions, and on the moderator variable and its effects.